Base electrodes for semiconductor devices



Sept. 2, 1952 H. B. LAW

BASE ELEcTRoDEs Foa szmcounucwoa vnEvIcEs Filed Allg. 31, 1949 f VWA i. l nventor I'Lfmnml B. LAW

'Gtfotneg Patented Sept. 2, 1952 DEVICE Earold B. LLaw,"Princeton, N. J., assignor totad'io g Corporation of `America, a corporation of `Delai- Ware nppiicatitn August e1, 1949, serial N. 113,233 i I ill Claims.

l .This 'invention relates generally 'to vsemi-conlduc'tor 'devices suitableas ampliers or `oscilla'- y tors, and :particularly 'relates jto novel bas/e electrod'e'sforsuch devices. y el l l "The *three-electrode ,senil-conductor fhas recantly' 'been developed as an amnliner Aorosei-11a'- tor. device, fwhichhas been termed a ftran'siston hasloen 'disclosed in Va series of three letters tothePhysic'al'RevieW by Bardeen and Brat'tainrattein andBardeen, and Shockley -lan'd ylffearson@whichfappear onfpages 230 to 233 othejlly 15,1948 issue.- 'The new .amplier. includes abbloo'l .oi ,ai'SeIi-conducting ma- 'aerial such as sil-icon lor germanium 'which is provided 'with two closely'adja'oent 'point electrodes called "emitte1" and lcolle'ctor" "electrodes in Contact with one surfaceregicn 'off the material, and ,a base 4electrode, VWhich 'provides largearea, .low-'resistance contacts vWith 'another surface region of the semifconducting' material. This amplilier .provides voltage as Wella-sourrent gainv under properlonrati'ng co'ndition'sand maybey consderedas `a lee-terminal network hailing a .ocnnnon.input'z and output l terminal.

Thus, thedevice 'is ele'ctivl'y a four-terminal' ,It is Usually A.as'SllX-ie'd that `13W()-el'eclfoce crystal. rectier-`requres 'a 'small-arcani' point electrode andanothe :electrode which contacts the 'bulk .o .the crystalgandlis of such large area that its, resistance'is. extremely low vfor either fdirectcn of current How. "Accordingly, 'non-linear eects atthisflargearea lectlode are vnOto 'tot and output elet:

great'signcanc compared to' those atlthe pointv electrode fhaving very fsiall area Which may be obtained ,by aliviehaifin'g a' sharp point. The actual rectificatntakes place at a hypothetical barrier-layer a't'thesracef ofthe crystal near the po'intelettrp such" of anNtvpesem; jon'ducto' will conduct readily Whenthe point 'electrode i:s'"p0'stive. in vpolarity and isQrelatively nonco'nducting `when the vpoint electrode'is madenegative. For a two-electrodey rectiiiervnade of' a P type semi-conducting crystal the situation is reversed.

By .analogy it hasjeenerauy'b" n assumed. that the baseeleotrojfle of aV three-electrode sencon' ductoi' amplinerjfmtist. be of large area, rn'u'st havev aicomparatively low contact' resistar'nzev 'and must be non'rectif'ying In accordance With the present inventionit has been Vfound that the? base electrode need Snot 4have a comparatively; large Contact area but that its cotfat area (ci. irs-ete)- mie, be the base'ele'c'- e9. elementen, ad t he iierstood "from 'th te of substantially :the same size that 'or the emitter vand `collector jele'ctodes vwhich are "point electrodes. Such 'a "small-area basefelectrode makes it possibleoto provide all three electrodes of a semi-conductorfanipliler on the :same 'surface of 'thecrystar This win facnitatethe lmanufacture of the "'device andf'yvll also jimn'roye' its high-frequency response Vfdife A'to the reduced transit time o"f the charge carrirswl'owin bei tween any twojo'f itselectrode's'. 'Stich angerrangement lhas 'the-'same elct as if 'the device hadv a very small :semi-conductingcrystal.

Itv 'is the principalobjectjof 'the present @inve'n tion therefore to provide n'o'velloaseelectrodesfor semi-conductorA devices s'itable as 'ainiilers ose 'tillatotsor'thelike.`- y v I l Anotherv object of 'the inv tioni's to `provide a hase electrodefor a `sem nondn'ctoiiarnblier or loscillator Whichhasjasmall contactfar'ea With the crystal comparableto that ofthe emitter and lcollector' electrodes Whichare rectifyingele' trades', whereby thevthreefelectrodes"cf"the H'evice may be providedon ithe'sanefsurface ofthe semi-*coi'iductingcrystal.y l A further kohjectfof theinve'ntion is to provide a vnovel vprocessfof pre u r g` alo'w1resistai1ce, non-'rectifying ei'eetrtefetontat'ting afsurface er a semi-conducting crystal.` Y", K

A semi-conductor ,ainpvliiier 'or oscillator. in ac cordance with the present Jiiivento'r'1v'ci`n`rripuises t a plurality `vof 'electrodesincludingfthebase ele'c trodejandhavin c ntaet'fre ywith 'the semi: conductingjcrys'ta n l Thus,the"ba se electrde-r'na y onsistlof l `'a drop ofV solder orvitr'n'ay consist ytra electrodep'sited layer of metalhavng avsr'nallw contact area-with' theV crystal. The loase electrode in this-oase haveY a low@resistancecontactv Withthf'crystal Alternatively, the base 'electrode may consist odork C cefmay consist il cornes either the when read in connection with the accompanying drawing, in which:

Figure 1 is a view in perspective of a threeelectrode semi-conductor device having a smallarea base electrode and embodying the present invention;

Figure 2r is a, sectional view of the base electrode and semi-conductor crystal of Figure 1;

Figure 3 is a sectional View of a modied base electrode and semi-conductor crystal in accordance with the invention;

Figure 4 is a View in perspective of a semiconductor device having three point electrodes and embodying the present invention;

Figure 5 is a graph illustrating the performance of a semi-conductor device having a soldered base electrode compared to that of a device having a point base electrode;

Figure 6 is a schematic circuit diagram of a semi-conductor amplifier referred to in explainl ing the operation of the device of Figure 4; and

Figure 7 is a graph illustrating current vs. voltage curves as well as the gain of the device of Figure 4. l

Referring now to the drawing in which like components have been designated by the same reference numerals throughout the figures, and particularly to Figures 1 and 2, there is illustrated a semi-conductor device embodying the present invention whichrmay be used as an amplier, oscillator or the like. The device comprises a block l of semi-conducting material consisting, for example, essentially of a chemical element having semi-conducting properties such as germanium, silicon, boron, tellurium, or selenium containing a small but sucient number of atomic impurity centers or lattice imperfections as commonly employed for. best results in crystal rectiers. Germanium is the preferred material for block IIJ and may be prepared so as to be an electronic N type semi-conductor crystal as is well known. The top surface of semi-conducting block I0 may be polished and etched in the manner explained in the paper by Bardeen and Brattain referred to. It is also feasible to utilize the germanium block from a commercial high-back voltage germanium rectifier such as the type 1N34.

The device of Figures 1 and 2 is provided with two point electrodes II and I2 which may be arranged or biased to be the emitter electrode and the collector electrode, respectively, of the device. Emitter electrode I I and collector electrode I2 may. for example, consist of a small-diameter wire of tungsten or phosphor bronze having a pointed end as shown. The contact areas of electrodes II and I2 are extremely small compared to the entire surface area of block I0. Thus, the diameter of the contact area of electrodes II and I2 may be of the order of l mil or less. Y

InA accordance with the present invention base electrode I is alow-resistance, small-area electrode-which is non-rectifying. Thus, base electrode I5 may be provided by fusing a drop of solder Aof a diameter of the order of 4 to 6 mils with the surface of germanium block I0 by means of solder iiux. As illustrated in Figure 2 Vthe drop of solder issomewhat at or oval shaped ldue to the surface tension between the crystal surface andthe solder. A solder suitable for base electrode I5 may, for example, have equal parts of tin and lead. vAny type of solder may be used for this purpose including Wood's metal which consists of 50 parts by weight of bismuth, 25 parts of lead, 121/2 parts of tin and 121/2 parts of cadmium. This type of solder has a melting point of 65.5 degrees centigrade.

The contact area of base electrode I5 is approximately 200 times smaller than the area of a conventional base electrode connected to a crystal which may have a square surface 45 mils long and wide. Nevertheless, the contact resistance of electrode I5 was found to be very low. Electrode I5 accordingly is a small-area, lowresistance, non-rectifying electrode.

The three electrodes II, I2 and I5, preferably are provided on the same surface of crystal ID as illustrated in Figure 1. Furthermore, the distances between the three electrodes II, I2 and I5 may be approximately equal and the electrodes may be arranged at the apices of an equilateral triangle as shown in Figure 1.

At the present time it is not possible to explain in detail the non-rectifying properties of a small-area solder electrode such as shown at I5. However, it is vbelieved that the solder diffuses into the topmost layer of germanium block I0. Consequently, no barrier layer exists between crystal I0 and electrode I5 so that the electrode is a non-rectifying electrode. Due to the low contact resistance between electrode I5 and the crystal block II) the potential of the bulk of crystal will be that of electrode I5. In other words, the resistance of electrode I5 should be low enough to control the crystal potential.

Electric contact may be made to solder electrode I5 by a thin wire such as shown at I6 in Figure l. This wire may easily be connected or soldered to the solder drop I5. It is also feasible to provide more than one base electrode such as shown at I5 in Figure l.

In accordance with the present invention it has also been found that a low-resistance, non-rectifying electrode may be provided by electrodepositing a layer of a suitable metal on a surface of crystal I0. Such an electrodeposited or electroplated layer is illustrated at I3 in Figure 3. This may be'accomplished as follows. Crystal I0 is rstpolished and etched in the conventional manner. Thereafter, the crystal is imbedded in wax except for a portion of the surface which it is desired to electroplate. This area preferably has approximately the same size as the contact area of solder drop I5, that is, it may have a diameter of approximately 4 to 6 mils. The crystal is now immersed in an aqueous solution containing 3.0 ounces of 'copper cyanide, 4.5 ounces of sodium cyanide and 2.0 ounces of so- Y dium carbonate per gallon of water. The crystal is made the cathode and a copper anode is placed into the bath. Electric current is allowed to flow between these two electrodes for a very short period of time of the order of 10 seconds wherebyj ing electrode. `However, itis believed that the` cyanide bathinayattack the previously etched surface of the crystalV and presumably the copper either diffuses into the crystal or forms a very intimate bond therewith. In any case, the contact resistance of electrodeposited layer I3 as measured between a point electrode and layer I8 or between ga conventional base electrodeand layer vI3 was foundto :be very low. The geometric arrangement of the three electrodes of asemiconductor amplier oroscillatcr may again be that ,illustrated in :Figure l-With theexception that solder drop l 5 is replaced `by -electrodeposited layer i8 of Figure 3. VvLayer i8 may be connected to a fine wire in the mannerpreviously explained.

n accordance with the present invention "it has been found that under Vcertain voperating conditions the ibase electrode need not even be 'a low-resistance, non-rectiying electrode. In other words, the base electrode may be a smal-larea electrode which `normally 'has a vhigh contact resistance with the semi-conductor crystal. Such a lolevlice'in accordance "with the present invention 4-is illustrated in Figure 4.V Semi-conductor block I0 is provided'with three pointelectrodes 20, 2i and 22. The three point electrodes may "consist of thin wires of tung-sten -or phosphor bronze,A 'for example, and Vmay either have a vpointed end as illustrated in Figure 4 or they may have a comparatively blunt end. Electrodes 2t to 22 accordingly have a relatively small contact area with crystal |f0 and are ordinarily rect-ifying electrodes. Electrodes 25 and'l may have a spacing of the order of `2 mils. As illustrated `in Figure 4, electrode 22 is `relatively remote from electrodes 20 and 2| and maybe spaced therefrom between approximately 20 and 25 mils. However, it lis to be understood that all three electrodes 20, 2| and 22 may be closely spaced to improve the high frequency response of the -device. I

In accordance with the present invention it has been found that the device illustrated in Figuree will show an appreciable gain if electrode 22 is made the baseV electrode while electrodes 20 and 2| are respectively emitter and collector yelectrodes. However, the gain which may be realized with the device of Figure 4 is not as high'as that which may be obtained with the device of Figure l. Furthermore, in accordance with this invention an amplifier having appreciableAg-ain is obtained if electrode-22 is madethe Vemitter electrode while electrodes 2t fand 2| may-bathe base and collector electrodes respectively. However, nogain is obtained if electrode 22 is made the collector'electrode While electrodes 2tV and2| are the base and emitter electrodes respectively.

Let it be assumed that electrode 22 is Ithe base electrode while electrode 20 is the-'emitterelectrode and electrode 2| v is the collector electrode. Forv operation as an amplifier it is conventional practice to provide a comparatively smallforward bias Voltage between the base and emitter electrodes while acomparatively large reverse bias voltage is impressed between the `coiiector and base electrodes. Accordingly, the base electrode 22 is biased in the forwarddirection -with respect to the collector electrode 2|. kThis simply means that base electrode 22 isbiased in such va manner as to have a relatively low `contactresistance compared to that-of the collector electrode 21|. Emitter electrode 20 is then also biased in a forward direction and accordingly alsohas a relatively low contact resistance compared to that of collectorv electrode 2|.

electrode 22 into vcrystal l0. Since a ypositive current flows from collector electrode 2i tdbase electrode 22 while a `positive, current ilows'from base electrode 22 /to emitter electrode 23,. a1net positive current will flow into base electrode 22 as long as the collector current is larger than This-is true as` long as a net positive current flows through base the .emittencurrent which correspondsto'tlie normalv operating conditions: of the; device.

vLAccordiI ie'ly,.it is believed that nnder these conditions thecontact resistance of Ybaserelec- ,-trodec22i'will vbe relatively lowand the device will function witha low-resistance,small-area base electrode. lt has also been found that the resistance.lookinggintobase electrode 22 ris approximately 'at itsinonnallowvalue when the collector current `isa maximum. However, as the emitter current increases the internalA base resistance also increases and may approach l7000 ohms.

Curve 2,5 of Figure 5 :illustrates the emitter currentiIe plotted against .the emitter voltage Ee for the device of Figure v4. The gain G in db (decibel) is illustrated by dotted curve 26. Curve 2J of Figure .5 illustratesthe emitter current Ie plotted against the emitter voltage E@ for a device having a soldered base `such as shown at i5 in Vligure 1. Curve v28 shownin dotted lines, illustrates the gain of a device having a solder base. It will readily be .seen that curves 25 and 2 Aare parallel. Thisindicates that the internal base resistance is approximately equal in the two cases because otherwise the .curves would diverge. Curves 25 and 21 are shifted by an amount corresponding to .36 volt which represent a contact lpotential difference which acts to oppose the voltage source provided between emitter'electrode 20 and base electrode 22.

Let it now be assumed that in the device of Figure Il, electrode 22 is made the emitter electrode while electrode 2G is the base electrode and electrode 2| the collector electrode. The device of Figure 4 has been illustrated schematically in Figure 6 as an ampli-iler with three point electrodes. Emitter electrode 22 has been shown inFigure 6 connected to a surfacefof crystal I0 which is opposite that to which collector electrode 20 and base electrode 2| are connected. This is intended to indicate schematically that emitter electrode 22 is relatively remote from collectorv electrode 20 and baseelectrode 2|. It is to be understood, however, that all three elecn trodes 2li, 2| and 22 are preferably arranged on the same surface of crystal Iii asshofwn in Figure 4.

ln the circuit of'Figure 6 battery 3d has its positive terminal connected to base electrode 2d while its negative terminal is connected to collector electrode 2| through resistor 3| indicating schematically a load impedance element. Battery 3|] isl accordingly connected in such a polarityas to apply a comparatively large reverse bias between collector electrode 2| and base electrode 20. Battery 30 has been indicated to be variable and its positive terminal may be grounded as shown. Battery 32 has its negative terminai connected to base electrode 20 while its positive terminal is connected through resistor 33 to emitter electrode 22. Battery 32 which has also been shown to be variable, is connected to apply a comparatively small forward bias between'base electrode 22i`and emitter electrode 22.k

The operation of the amplier indicated in Figure 6 is'not clearly understood at the present time. However, it is believed that due to the applied bias voltages the contact resistance of emitter electrode 22 andthat of thebase elec trode 20 are relatively low compared to the contact resistanceofcollector lelectrode 2|. it is further `believed that base electrode 253, which is relatively close to collector electrode 2|, will emit holes which iowy through crystal it to collector electrode 2| where they-are collected. It is assumed that this mode of operation is predicated on the fact that base electrode has a contact resistance which is not negligible. In other words, base electrode 2li does not appear to control the potential of the crystal because presumably, if it did, it would normally not be able to emit holes. It is accordingly believed at the present time that the amplifier of Figure 6 functions due to the fact that base electrode 20 will emit holes and therefore effectively functions as an emitter electrode.

Since a semi-conductor amplifier having an N type semi-conducting material functions by reason of the fact that holes are emitted by the emitter electrode and collected by the collector electrode it can readily be seen why the device cannot operate if the remote electrode 22 is made the collector electrode. This is so because the holes will only travel over a relatively short distance of the order of a few mils and cannot be collected by a collector electrode remote from either the emitter electrode or from an electrode which effectively emits holes.

Curve 35 of Figure 'I illustrates the collector current Ic plotted against emitter voltage Ee for the amplifier of Figure 6. Curve 35 shows the emitter current Ie plotted against the emitter voltage Ee while curve Si indicates the gain as a function of the emitter voltageY Ee. The gain has a maximum at Erri-.2 volts. When the emitter voltage is approximately -i-.4 `volt the collector current Ic is comparatively small but larger than the emitter current Ie. This seems to indicate that electrons are leaving and holes arriving at collector electrode 2l. As Ee decreases towards zero the hole emission presumably increases and consequently more electrons leave collector electrode 2l. Hence, both It and I@ increase. As the emitter electrode 22 becomes negative the electron current Ie begins to decrease but the hole emission continues to increase as evidenced by curve 35. Curves 35 and B can be replotted against the voltage of electrode taken with respect to the voltage of 'electrode 22. In that case the difference between curves (It) and 36 (Ie) which represents the base current will be similar to curve 2l in Figure 5.

A semi-conductor device having three point electrodes has the advantage of simplified construction. Furthermore, all three electrodes may be provided on the same surface of the crystal which will improve the high frequency response of the device particularly if the three electrodes are close to each other.

There has thus been disclosed a semi-conductor device suitable as an amplifier, oscillator or the like having novel base electrodes. The base electrode in accordance with the invention is a small-area electrode which may have a low contact resistance and may be a non-rectifying electrode. Such a base electrode may be provided by a drop of solder or by electrodepositing a small-area metallic layer on the crystal. Alternatively, the base electrode may under certain conditions be a point contact which is normally a high-resistance, rectifying electrode.

What is claimed is:

1. A semi-conductor amplifier or oscillator device comprising a semi-conducting body, and a plurality of electrodes in contact with said body, one of said electrodes being the base electrode, said base electrode having a contact resistance with said body which is lower than that of the 8 other ones of said electrodes, whereby said base electrodecontrols the potential of the bulk of said body, the contact areas of said electrodes with said body having substantially equal size.

2. A semi-conductor amplifier or oscillator device comprising a semi-conducting body, and three electrodes only in contact with said body, one of said electrodes being the base electrode. the contact areas of said electrodes with said body having substantially equal size.

3. A semi-conductor amplifier or oscillator device consisting of a semi-conducting body having a substantially fiat surface, and three substantially equally spaced electrodes only in contact with said surface, one of said electrodes being the base electrode, the contact areas of said electrodes with said surface being substantially equal in size.

4. A semi-conductor device suitable as an ampliiier or oscillator and consisting of a semi-conducting body, an emitter electrode, a collector electrode and a base electrode in contact with said body, the contact areas of said electrodes with said body being substantially equal and small compared to the surface area of said body.

5. A semi-conductor device suitable as an amplifier or oscillator and consisting of a semiconducting body having a substantially flat surface, an emitter electrode, a collector electrode and a base electrode in contact with said surface, the contact areas of said electrodes with said surface being substantially equal and small compared to the area of said surface.

6. A semi-conductor device suitable as an amplifier or oscillator and comprising a semi-conducting body, and three point electrodes including a base electrode in contact with said body, one of said electrodes being remote from the other two electrodes, said other two electrodes being located relatively close to each other, the contact areas of said electrodes with said body being of substantially equal size.

7. A semi-conductor amplifier or oscillator device comprising a semi-conducting body, and three point electrodes in contact with said body, one of said electrodes being remote from the other two electrodes, said remote electrode being the base electrode, the contact areas of said electrodes with said body having substantially equal size.

8. A semi-conductor amplifier or oscillator device comprising a semi-conducting body, and three point electrodes in contact with said body, one of said electrodes being remote from the other two electrodes,rsaid remote electrode being the emitter electrode, the contact areas of said electrodes with said body beingv of substantially equal size.

9. A semi-conductor amplifier or oscillator device comprising a semi-conducting body, two rectifying high-resistance electrodes in contact with said body, and a non-rectifying low-resistance electrode in small-area contact with said body, said non-rectifying electrode consisting of solder, the contact areas of said electrodes being substantially equal.

l0. A semi-conductor device comprising a semi-conducting body, a non-rectifying electrode in contact with said body, said electrode-consisting of an electrodeposited metallic layer having a contact area with said body which is small compared to the surface area of said body, and two further electrodes in rectifying contact with said body, the contact areas of said elec- REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number l0 Number Name Date Weber Feb. 27, 1934 Rack July 19,1949 Pearson et al. Apr. 4, 1950 Shockley May 15, V1951 Pearson July 17, 1951 FOREIGN PATENTS Country Date Great Britain Sept. 25, 1945 

